The presence of large deposits of oil shale in the high plateau, semi-arid region of the western United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is, in fact, a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen" which, upon heating, decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents such as U.S. Pat. Nos. 3,661,423; 4,043,597; 4,043,598; and 4,192,554; and in U.S. patent application Ser. No. 070,319 filed Aug. 27, 1979, now abandoned, by Chang Yul Cha, entitled TWO-LEVEL, HORIZONTAL FREE FACE MINING SYSTEM FOR IN SITU OIL SHALE RETORTS. Each of these applications and patents is assigned to Occidental Oil Shale, Inc., assignee of this application, and each is incorporated herein by this reference.
These patents and applications describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded to form a stationary fragmented permeable mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort, or merely as a retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone in the retort and introducing an oxygen-supplying retort inlet mixture into the retort to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. By the continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting". Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbons, and a residual carbonaceous material.
The liquid products and the gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off-gas is also withdrawn from the bottom of the retort. Such off-gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process.
U.S. Pat. Nos. 4,043,597; 4,043,598; and 4,192,554 disclose methods for explosively expanding formation containing oil shale toward horizontal free faces to form a fragmented mass in an in situ oil shale retort. According to such a method, a plurality of vertically spaced apart voids are initially excavated, one above the other, within the retort site. A plurality of vertically spaced apart zones of unfragmented formation are temporarily left between the voids. Additionally, a plurality of vertical blastholes are drilled into each zone of unfragmented formation. The blastholes are loaded with explosive, forming an array of vertical columnar explosive charges in each zone of unfragmented formation. The explosive charges are then detonated to explosively expand each unfragmented zone upwardly and/or downwardly towards the void(s) above and/or below it to form a fragmented mass having an average void volume equal to the void volume of the initial voids. Retorting of the fragmented mass is then carried out to recover shale oil from the oil shale.
Blastholes formed in unfragmented formation can, for example, be 10 to 12 inches or even more in diameter and each explosive charge formed in the blastholes can have a length about one-half the thickness of the zone of unfragmented formation being expanded.
The zones of unfragmented formation being explosively expanded can be tens of feet thick. For instance, in U.S. Pat. No. 4,192,554, a zone of unfragmented formation 35 feet thick is explosively expanded toward a void. In this embodiment, zones of unfragmented formation provided are 160 feet long and about 160 feet wide with a total of 81 blastholes formed in such a zone. Therefore, 81 explosive charges which have a length of about 171/2 feet are formed in the zone of unfragmented formation.
Detonating explosive in a single round in such a zone of unfragmented formation produces a powerful explosion which generates seismic shock waves travelling outwardly through unfragmented formation extending away from the blasting site.
Seismic shock from such a powerful explosion can cause rock falls and serious damage to equipment and structures, such as bulkheads and piping, in nearby underground workings. Also, equipment and buildings located nearby above ground can be damaged by such detonation.
Thus, it is desirable to provide a method which provides a desired amount of fragmentation, while controlling, i.e., minimizing, seismic effects at locations near the blasting site.